The invention relates to a method and a device for measuring a folding angle of a sheet, and to a folding machine.
From EP -A 715 552 a method is known for optimizing the folding of a sheet between a punch and a die of a folding machine, to a desired folding angle. During the folding of the sheet, the folding angle of the sheet is measured and the movement of the folding punch is controlled by an adjustable controller as a function of the measured folding angle. The folding angle is measured by a tracer placed in a reference position. The tracer comprises a telescopic device which can be placed against a point on the underside of the sheet and a distance sensor for measuring the position of the telescopic device. The folding angle is determined as a function of the signal of the distance sensor. This method requires a calibration using at least one sample piece with a known folding angle to determine the relationship between the signal of the distance sensor and the folding angle. Furthermore, the tracer must be placed very precisely in the reference position.
From EP -A 470 263 a method is known in which the folding angle is measured by a vision system. This vision system comprises a plane light source and an image pickup device for picking up an intersection line pattern of light induced onto the surface of the sheet by the plane light source. The vision system is placed in a reference position in accordance with the co-ordinate axis of a co-ordinate plane. The folding angle of the sheet is calculated based on the positional relationships between the pick-up direction, the direction of the emitted light, the intersection line and the co-ordinate axis of the co-ordinate plane. This method requires a precise attachment of the vision system with respect to the sheet and a calibration using a sample piece of a known angle. Furthermore, this vision system has to be attached a well-defined distance away from the sheet, thereby enabling the emitted light to reach the focal point of the image pickup device. This can lead to the use of a non-standard support die or punch, with the vision system built in.
The aim of this invention is a method and a device for measuring a folding angle of a sheet without the need for a precise attachment or a precise positioning of the device according to the invention with respect to the sheet and/or the folding machine.
To this end, the method according to the invention comprises the measurement, on at least one side of an element, of a number of distances in a plane that crosses the sheet and the element; said distances comprising a number of distances between a measuring tool and different points on the sheet and a number of distances between a measuring tool and different points on the element; determining a distance profile of the measured distances; determining two straight lines based on values of the distance profile, and determining the angle between sheet and element from the angle between the two straight lines, so as to determine the folding angle of the sheet as a function of the thus determined angle between the sheet and the element.
The method according to the invention offers the advantage that the folding angle of the sheet can be determined irrespective of the position of the measuring tool relative to the sheet. This method further offers the advantage that the folding angle can be determined very accurately, even if the measured distances were not measured very accurately. Because of these advantages, the method according to the invention is very suitable for measuring a folding angle of a folding sheet in a folding machine.
In a preferred embodiment, the method comprises the measurement, on two sides of an element of a number of said distances in a respective plane that crosses the sheet and the element; determining, for each side of the element, a respective distance profile of the measured distances, determining, for each distance profile a set of two straight lines, based on values of the distance profile, and determining a respective angle between sheet and element from the angle between the straight lines of each set, so as to determine the folding angle of the sheet as a function of the two determined angles between the sheet and the element.
In a preferred embodiment, the method comprises the measurement of a number of said distances in a plane that crosses the sheet and the element perpendicular to the longitudinal direction of the sheet and the element. This offers the advantage that no calibration is necessary in order to determine the angle between sheet and element.
Preferably, the folding angle is measured during the folding of the sheet between two elements in a folding machine.
In an embodiment, a distance profile of the measured distances is determined as a profile in a co-ordinate system with respect to the measuring tool.
In another embodiment, the two straight lines are determined statistically, based on values of the distance profile.
The device for measuring a folding angle of a sheet according to the invention comprises a measuring tool for measuring, on a side of an element, a number of distances in a plane that crosses the sheet and the element, said distances comprising a number of distances between the measuring tool and different points on the sheet and a number of distances between the measuring tool and different points on the element; means for determining a distance profile of the measured distances; means for determining two straight lines based on values of the distance profile and means for determining the angle between sheet and element from the angle between the two straight lines so as to determine the folding angle of the sheet as a function of the thus determined angle between the sheet and the element. A device according to the invention can be used to measure a folding angle of a sheet in a folding machine and will not impede the folding of the sheet in the folding machine, since all parts of the said device can be arranged far enough away from the support die and the punch. In a preferred embodiment, the device comprises means for mounting the measuring for measuring a number of distances in a plane that crosses the sheet and the element perpendicular to the longitudinal direction of the sheet and the element.
In a preferred embodiment, the measuring tool comprises a scanner that is rotationally supported and comprises a means for controlling the rotational position of the scanner, in order to determine in each rotational position of the scanner the distance between the scanner and the sheet or the distance between the scanner and the element.
In a preferred embodiment, the element is an integral part of a folding machine. Preferably, the element consists of a support die for supporting the sheet in a folding machine.
The folding machine according to the invention comprises at least one device according to the invention, said device being mounted on a side of an element of the folding machine. Preferably, the folding machine comprises two devices according to the invention, said devices being mounted on different sides of an element of the folding machine. Preferably, the element consists of a support die for supporting the sheet in a folding machine.
A folding machine according to the invention offers the advantage that the folding angle of a sheet can be measured easily during the folding of the sheet, and that the folding machine can be equipped with a standard support die and a standard punch.